It's personal preference first I guess, but like many here I remain undecided as to what's the "best" for my needs.
For sure I often side with my 13" MBP, screen is beautiful, clear, resolution is great on that notebook, (better than it's hi-res competitors), small, portable, so easy to take everywhere .... so there's no issue there, except ...
I spend so many hours a day, everyday staring at a notebook screen it would/does make more sense to use a larger screen, or the largest screen I can handle, right? After all it's your 'window' to the internet, bigger should be better, I don't travel much so even a 17" would work, although I'm sure I'd find it large on the lap, coffee shop, patio etc.
15" should be a good compromise for me then. Right? While I really enjoy the 15" screen I can't help but think the notebook still seems large. I don't feel I need a screen larger than a 13" most days, I don't like having multiple web pages open in front of me, most often there's no need for what I do, and you can only concentrate on one thing at a time anyway, right?
There's arguments for and against each screen size ... I even have a 24" ACD, but I find myself most comfortable with 13" in hand, on the go, or on the couch, it's always there. The 15" I think twice of moving from the table or bringing it will me, it is heavier, larger, clunkier...
I wouldn't be surprised if smaller laptops/notebooks gained market share and became more popular. Personally, I prefer a 13" screen or larger, anything under that, for all day, working often, is too small. I wish the iPad was a super thin clam shell design with a physical keyboard and more work oriented/input capabilities.
So my guess is the iPads, netbooks of the world will gain in popularity because of their 'take everywhere' design but as 12/13" notebooks become thinner, cheaper, more capable people will gravitate to the larger screen, OR before too long devices like the iPad will (wirelessly) connect to an external display, keyboard and have more processing power, then the game will have changed forever............